The invention relates generally to radiation detectors. In particular, the invention relates to radiation scintillator detectors.
Radiation scintillator detectors are known in the well drilling industry for logging and measure while drilling (MWD) applications. Radiation scintillator detectors may also be used as security portal detectors or in medical applications. When the radiation scintillator detector is incorporated into a logging tool of a tool string used for the drilling of oil, gas and water wells, the logging tool identifies, locates and differentiates geologic formations along a well bore. Tool strings and logging tools for oil wells are often exposed to harsh operating environments including temperatures that can reach 185° C. and pressures up to 20,000 psi and can be exposed to severe shock and vibration.
A typical radiation detector assembly includes a scintillator coupled to a light detecting element such as a photomultiplier tube. Radiation, such as gamma rays emitted by geologic formations, is converted to light by the scintillator and conducted to the photomultiplier tube. The photomultiplier tube converts the light into electrons and produces an amplified electrical signal. The amplified electrical signal is then measured and used by monitoring electronics. It is desirable to have the amplified electrical signal produced by the photomultiplier tube, in the absence of noise, to be directly proportional to the gamma rays interacting in the scintillator that are converted to light.
Any “noise” component in the amplified electrical signal could lead to a misrepresentation of the gamma rays reacting in the scintillator. “Noise” or dark current, in the form of an electrons produced in the photomultiplier tube, can be created by thermal activity and not by a photoelectric effect. These electrons are known as thermionic electrons. The amplified electrical signal produced in the photomultiplier tube that include thermionic electrons, could misrepresent or distort the signal created by gamma rays deposited in, reacting or interacting with the scintillator. Operating the photomultiplier tube at high temperatures increases the emission thermionic electrons and noise levels. For certain applications, such as in downhole drilling, an accurate measure of a geologic formation's background radiation is required and spurious electron production from “noise” can affect the performance of the radiation detector assembly. It is, therefore, advantageous to eliminate or minimize the “noise” component in the radiation detector assembly.